The oxidative dehydrogenation of ethane to acetic acid in the gas phase is well known in the art. Generally, this process involves reacting a gaseous feed in a fluidized bed or in a fixed-bed reactor. The gaseous feed comprises ethane and/or ethylene which are fed to the reactor as pure gases or in admixture with one or more other gases. Examples of such additional, or carrier, gases are nitrogen, methane, carbon monoxide, carbon dioxide, air and/or water vapor. The gas comprising molecular oxygen can be air or a gas comprising more or less molecular oxygen than air, e.g. oxygen. Relatively high oxygen contents are preferred since the achievable ethane conversion, and thus the yield of acetic acid, is higher. Oxygen or the gas comprising molecular oxygen is preferably added in a concentration range outside the explosive limits under the reaction conditions since this makes the process easier to carry out. However, it is also possible to employ an ethane/ethylene to oxygen ratio within the explosive limits. The reaction is carried out at temperatures of from 400 to 600° C., while the pressure can be atmospheric or superatmospheric, e.g. in the range from 1 to 50 bar.
Ethane is usually first mixed with the inert gases such as nitrogen or water vapor before oxygen or the gas comprising molecular oxygen is fed in. The mixed gases are preferably preheated to the reaction temperature in a preheating zone before the gas mixture is brought into contact with the catalyst. Acetic acid is separated from the gas leaving the reactor by condensation. The remaining gases are recirculated to the reactor inlet where oxygen or the gas comprising molecular oxygen and also ethane and/or ethylene are metered in. The recirculated gases will always comprise both ethylene and ethane.
FIG. 1 shows a common prior art acetic acid production process. In this basic system, an ethane containing stream (1) is fed along with an oxygen containing gas (2) into an ethane oxidation reactor (3). This reactor can be either a fluidized bed or a fixed-bed reactor. Inside the reactor (3), ethane is oxidized into acetic acid, ethylene, and various carbon oxides (COx). The gaseous reactor effluent (4) that contains these three primary components is fed into a recycle gas scrubber (5), which produces a top stream containing ethylene, ethane, and COx. The top stream (7) from the recycle gas scrubber is routed to a processing step (8) that removes the COx from the top stream. The purified stream (9) is then recycled to the oxidation reactor (3) for further conversion into acetic acid. The bottom stream (6) from the recycle gas scrubber (5), which contains acetic acid, water, and heavy ends by-products, may be purified as known in the art to provide purified acetic acid. For example, the bottom stream may be routed to a drying column to remove water followed by a heavy ends column to remove propionic acid and other heavy components.
High Ethylene content in the recirculated gas stream will lead to poor efficiencies and higher losses to the COx byproduct in the acetic acid reactor. In fact, ethylene in the recycle feed competes with the ethane for the limited amount of oxygen feed, resulting in higher inefficiencies to carbon oxides than straight ethane oxidation. It is therefore desirable to develop a process where there is no ethylene in the recycle stream to the ethane oxidation reactor.